You add it to the remote system. I use it for updating a symlink in a known location to point to SSH_AUTH_SOCK. I then get tmux sessions set up to look in that

You add it to the remote system. I use it for updating a symlink in a known location to point to SSH_AUTH_SOCK. I then get tmux sessions set up to look in that location. Effectively, each time I ssh in, I fix all the ssh forwarding in active sessions (this is on local dev vagrant machines).